Article for encapsulating expandable objects

ABSTRACT

A fabric cover (9, FIG. 2a), which is shrinkable at least in part, for encapsulating at least part of an expandable object such as an automotive airbag (1), the fabric cover being provided with a predetermined region of weakness (13), preferably a catch thread holding the edges of a woven fabric together to form a tubular or flat cover, which region of weakness will rupture when the said object expands by greater than a predetermined amount, e.g. in an automobile accident. The region of weakness (13) and preferably fabric regions (11) immediately adjacent thereto may be of a contrasting color to facilitate alignment of the region of weakness in the direction of expansion of the expandable object.

This invention relates to a cover for encapsulating an expandableobject, and particularly to a cover for encapsulating a vehicular airbag.

Airbags are commonly provided in vehicles to protect the occupants inthe event of an accident. A vehicular airbag typically comprises agas-inflatable bag, which is retained in its collapsed state within ahousing, or other container, in normal use, but which is inflated, andbursts out of its housing in the event of a sudden impact, as wouldoccur, for example, in an accident. An airbag may typically be providedin the car steering wheel, to protect the driver against front impact.Similarly an airbag may be provided in the front dashboard or glove box,on the passenger's side of the vehicle, to protect the front passengeragainst front impact. More recently, it has been proposed also to mountairbags in, or around, the doors of vehicles, or in or around the sidesof seats, to protect the occupants of the vehicles from side impact.Such air bags are described, for example, in an article entitled "SideImpact Airbag Systems" in "Automotive Industries" Febuary '95, at page104.

The cover of the present invention is suitable for encapsulating interalia any of the known types of vehicular airbags, or any otherconceivable vehicular airbag e.g. rear impact airbags, and airbags usednot only internally, but also externally of vehicles.

An important property of any housing, or other container, for avehicular airbag is that it must open reliably and instantly, preferablyin milliseconds, in the event of an impact, in order to allow deploymentof the airbag. It is also usually desirable that the airbag housingopens in a predetermined position in order to control the direction ofdeployment of the airbag. It is known in the art to achieve both theseobjectives by providing an airbag housing with a pre-determined tearseam, which is weaker than the main body of the airbag housing, andwhich opens, in the event of impact, to deploy the airbag.

EP-A-0604776 and GB-A-227086, for example, describe airbag housingswhich are provided with thinner walled sections, or grooves, which actas a line of weakness, and tear open to release the airbag, on inflationof the airbag. Shrink-fitted sheaths having such a line of weakness aredescribed in DE-A4137691 (Mercedes-Benz). Similarly, U.S. Pat. No.5,288,103 describes an airbag cover which is made from a firstthermoplastic material, but which contains an opening filled with asecond thermoplastic material of lower tensile strength and elongationstrength than the first thermoplastic. The filled opening is provided inthe shape of the desired tear-line, and in the embodiments shown is inthe shape of an "H".

EP-A-0510738 describes another approach to containing airbags, whichdoes not use predetermined tear lines formed in the housing. InEP-A-0410738, an inflatable gas bag is contained in a steel housinghaving an open side facing the dashboard. The open side is covered by ahinge flap which can swivel upwards in the direction of the windscreenwhen the air bag inflates. The open side is covered by a continuous,thin (50 microns), film of polyethylene or polypropylene shrunk aroundthe housing. The film may surround the entire housing, or cover only theopening. On the exterior of the housing, on the section facing thewindscreen, a metal strip with saw teeth, or a tooth shaped protrusion,is provided to perforate the thin film, when the air bag inflates.

We have found that a particularly effective container for a vehicularair bag may be provided by a heat-shrinkable fabric cover provided witha predetermined region of weakness, preferably a catch thread ashereinafter described.

The present invention therefore provides a fabric cover, which isshrinkable at least in part, for encapsulating at least part of anexpandable object, the cover being provided with a predetermined regionof weakness, which will rupture when the said object expands by greaterthan a predetermined amount

The invention also provides a method of encapsulating at least part ofan expandable object, the method comprising installing such a shrinkablefabric cover over the said part of the expandable object to restrain thesaid object in a first, compressed, state.

The cover and method according to the present invention find particularapplication for encapsulating a vehicular air bag. However they may beused for encapsulating any expandable object, particularly where it isdesired to constrain and/or compact the object, and also controldeployment of the object. This patent specification will refer primarilyto the use of the cover for containing vehicular airbags, but allcomments made are also applicable to containing other objects. Asexamples of other applications, for the cover of the present inventionthere may be mentioned the encapsulation of various inflatable objectssuch as buoyancy aids, inflatable bags for use for emergency lifting,inflatable protective packaging or clothing, and the like, and theencapsulation of objects that expand, or are expanded by a foamingmaterial. The cover may also be used to cover objects that arethemselves deployed by a backing expandable object. For example oxygenmasks in aircraft may be covered by a cover according to the invention,and then deployed by the action of a backing expandable object e.g. afoam, which expands in response to a trigger stimulated in an emergency.Other applications of the cover according to the invention would beapparent to the man skilled in the art.

By "expandable object" we mean any object which changes from occupying afirst smaller volume of space to a second larger volume of space. Asexamples, the expandable object may be in a furled configuration, in itsfirst, smaller volume state, and expanded by unfurling. The furled stateof the expandable object is preferably uniform. For example the object,such as an airbag, may be generally planar and arranged in a spirallyrolled configuration, or a pleated configuration within the cover.Instead, or in addition the expandable object may comprise an expandablematerial, e.g. a foam which can expand from its smaller first volumestate.

The present invention also provides a cover according to the inventioninstalled on an expandable object prior to its expansion.

The cover of the invention ruptures when the expandable object expandsby greater than a predetermined amount. Thus a slight increase in thevolume occupied by the expandable object will not cause rupture of thecover. Preferably the cover ruptures when the expandable object expandsto occupy a volume that is at least 120%, preferably at least 150% oreven at least 200% of its unexpanded state.

The article according to the present invention uses a shrinkable, cover.Preferably the shrinkable cover is heat-shrinkable. Heat-shrinkablearticles are well known. They may shrink, on heating, towards anoriginal shape from which they have previously been deformed, or theymay shrink to adopt a new configuration, even if they have not beenpreviously deformed. Reference may be made, for example, to U.S. Pat.No. 2,027,962, U.S. Pat. No. 3,086,242, U.S. Pat. No. 3,597,372, andGB-A-1440524.

Known heat shrinkable articles include plain polymeric materials e.g.polyolefin sheets, fabric covers, and composite sheets comprising fibersor fabrics. In the composite sheets, a fabric comprises polymericfibers, optionally also in combination with non-polymeric fibers such asglass, are typically laminated with a polymeric matrix, and either thefibers, or the matrix, or both, may be heat shrinkable to render thecomposite sheet heat shrinkable. Examples of heat shrinkable compositesheets are described, for example, in EP-A-0116393, EP-A-0117026.

Heat shrinkable fabric articles without a laminating polymeric layer arealso known. U.S. Pat. No. 3,466,210 (Wareham), for example, describesthe use of a woven or braided sleeve which has a heat shrinkable fiberinterlaced in the sleeve in the direction of required shrinkage, for useas a protecting layer on an article such as an armature for an electricmotor. Another composite sheet is described in U.S. Pat. No. 3,058,863(Gaines). This describes manufacture of a packaging material, for heavyduty purposes such as tool packaging, comprising laminating polyethylenefilm to a fabric comprising essentially polyalkylene fibers.

In the production of heat shrinkable articles, the polymeric material(of the sheet, and/or of the fibers in the case of a composite sheet) isusually cross-linked at some stage in the production of the article, toenhance the dimensional shrinkability.

The shrinkable fabric covers used in the article according to theinvention may be plain fabric or composite fabric with a polymericmatrix. The covers used in the invention are preferably cross-linked, orcomprise cross-linked components.

The shrinkable cover of the present invention may surround the entireexpandable object, or only part of the object. For example, the covermay be provided in the form of a tube around the air bag, e.g. forencapsulating a side impact vehicular air bag, and thereby completelysurround the air bag. Instead, the cover may be used in conjunction withan open sided housing, the cover being shrunk over the open side of thehousing, e.g. for encapsulating a front impact airbag. In bothapplications, the heat shrinkable nature of the cover may advantageouslybe used, not only to contain, but also to compact the airbag. This maybe advantageous for all types of air bags, but is particularlyadvantageous for side impact air bags that are to be installed in thedoor frame or the side frame of the vehicle, or the sides, or otherparts, of passenger seats where storage space is limited.

In order to enhance the compaction capability of the heat shrinkablecover of the present invention, the cover preferably has a recoveryratio of at least 1.2:1, preferably at least 2:1. In addition the coverpreferably has a thickness of at least 0.08 mm, preferably at least 0.1mm, more preferably at least 0.2 mm, especially at least 0.4 mm. Thisthickness allows the cover to compact the airbag, without itselftearing. In the arrangement of EP-A-0510738, the cover merely covers theopen side of the housing, and does not act to compact the airbag.

Throughout this specification preferred values of recovery ratios aregiven sometimes as a numerical ratio, e.g. 2:1, and sometimes as apercentage value, e.g. 20%. Where a numerical ratio is used for recoveryratio, this refers to the following ratio: ##EQU1## Where a percentagevalue is used for recovery ratio this refers to the following ratio:##EQU2## Any dimension of the article may be measured, since the figureis a ratio, e.g. the diameter or circumference of the tube may bemeasured for a radially recoverable object. By "free recovery" is meantrecovery without any constraint.

The heat shrinkable covers according to the present invention preferablycomprise cross-linked materials. For example, in a polymeric fabric, thepolymeric fibers making up the fabric may be cross-linked and/or thepolymeric sheet or matrix of a composite sheet may be cross-linked. Theinvention does, however, envisage the use of non cross-linked covers forcertain applications, particularly where required recovery ratios arelow, for example less than 20%.

In order to enhance further the compaction capability of the cover,where the shrinkable cover is a cross-linked heat shrinkable cover, thecover preferably exerts a recovery force of at least 0.1 MPa, morepreferably at least 0.5 MPa, especially preferably a recovery force ofat least 1.5 MPa when measured after 4 minutes fully held out in thejaws of a tensile testing machine at a temperature of 30-80° C. abovethe crystalline melting point of the shrinkable component of the cover.

The cover preferably has the appropriate properties of thickness,recovery ratio, and recovery force, so that it can be installed on anair bag, so that in its compacted state the volume occupied by the airbag is at most 80%, more preferably at most 70%, especially preferablyat most 60%, 40% or 20% of its pre-compacted volume.

In addition to enhancing its compaction capability, the minimumthickness of the heat shrinkable cover of at least 0.08 mm, also has anadvantage in reducing noise from the air bag during motion of thevehicle. With prior art air bags, of the type described for example inEP-A-0510738, noise caused by rustling of the covering film and air baghas been a problem. Use of a fabric-based cover substantially eliminatesthis noise. Without in any way limiting the invention, it is thoughtthat fabric-based covers are particularly effective in minimizing noisewhen used to enclose an expandable object such as an airbag, because ofthe inherent porosity of fabrics.

Furthermore, thin films, of the type described in EP-A-0510738 havesuffered from the problem of shattering into pieces when impacted by theair bag. The small pieces of the film thrown into the vehicle on impact,may cause injury to the passengers. This problem may also be alleviatedby using the thicker fabrics containing a region of weakness accordingto the present invention.

The cover according to the present invention is provided with apredetermined region of weakness. This region may be in a number ofshapes and be provided in a number of ways. For some applications theregion of weakness is preferably a line of weakness, which may bestraight or curved, or any combination thereof.

For the fabric-based shrinkable covers, the line of weakness isadvantageously provided by appropriate fabric design. In a preferredembodiment a catch thread is provided to join abutting edges of one ormore heat recoverable fabric covers. The catch thread is arranged torupture when the air bag, or other expandable object, is inflated, orotherwise expanded, and thereby provide a reliable line of weakness.Rupture preferentially at the catch thread region may be effected bymaking the catch thread of a weaker material than the fiber componentsof the fabric cover, by making it thinner, e.g. of lower tensilestrength and/or lower elongation to break, at least in regions along itslength, than the fibers of the fabric, or by the design of the catchthread interengagement with the edges of the fabric cover, or by anycombination thereof. The fabric is preferably woven, but other possiblefabric designs include knitted, braided, and non-woven designs.

For certain applications, as hereinbefore mentioned, the cover ispreferably generally tubular. Where the cover is tubular, the tube maybe closed in cross-section or open, i.e. wraparound. Tubularfabric-based covers of closed cross-section may conveniently bemanufactured by tubular weaving, needle insertion weaving, knittingetc., or by wrapping a flat sheet into a tube, and closing, e.g. bysewing or welding, overlapping or abutting longitudinal edges.Wraparound fabric based covers are conveniently manufactured as flatsheet.

Where the cover is a tubular woven fabric, the predetermined region ofweakness is preferably provided by a catch thread which extends as oneof the warp yarns extending in the longitudinal direction of the tubularcover. The catch thread preferably engages weft yarns which extend inthe circumferential direction of the tubular cover, and which then loopback on themselves around the said catch thread. Such a fabric designmay conveniently be made by needle insertion weaving.

Where a catch thread is used, the tensile strength of the catch threadis preferably in the range 3 to 5 Newtons, preferably about 4 Newtons,and the elongation to break of the catch thread is preferably in therange 15 to 25%, preferably about 20%.

Fabric constructions may conveniently be modified to vary the mechanicalproperties across the sheet. For example, the fiber density at edges ofa fabric sheet may conveniently be increased to increase the strength ofthose edges. This may be advantageous, for example, where the coveredges are to be anchored to a vehicle frame, or, as another example, toprovide reinforced edges for ease of sewing, or other joining means

As another possibility a flat fabric-based cover may be wrapped around agenerally elongate object such as an airbag, then sewn, or otherwisesecured in place around the expandable object, prior to shrinkage of thecover.

In preferred embodiments the region of weakness, and/or the section ofthe cover which immediately surrounds the region of weakness of thecover, is a different color, or otherwise easily distinguished, from theremainder of the cover. In one embodiment, most of the cover is onecolor, and the region of weakness, and/or the section of the coverimmediately surrounding the region of weakness, is a different color.This makes it particularly easy to see the region of weakness. This maybe advantageous during installation of the cover where it is desiredthat this region is oriented in a particular direction. For example, fora passenger protection vehicular airbag cover, the region of weaknessshould point towards the passenger(s) to be protected. Where afabric-based cover joined by a catch thread is used, the catch thread ispreferably a different color from the other fibers of the fabric, and/orthe fibers of the fabric cover immediately adjacent to the catch threadare a different color from the remaining body of the fabric, e.g.forming easily visible bands or stripes which readily identify thelocation of the catch thread.

For other applications the cover is preferably generally planar. Forfabric-based covers, these may initially be manufactured in planar form,or they may be manufactured as a tube (joined by a catch thread) andthen slit longitudinally at a point away from the catch thread, to forma sheet with a line of weakness extending along its length.

A number of designs are possible for interengagement of a catch threadwith the edges of a fabric shrinkable cover. A particularly preferredarrangement for joining edges of a woven fabric is to use a catch threadin a needle loom selvedge system of the type described in FIG. 1 of BS(British Standard) 7141: Part 1 1991. The British Standard describes alocking thread selvedge system for retaining the weft fibers at theselvedge of a single woven fabric. It would be obvious to the skilledman how to use this selvedge locking thread as a catch thread to jointogether looped weft fibers of two abutting woven fabric edges. Thespecific design of catch thread equivalent to the quoted BS lockingthread is illustrated in the drawings of the present specification.Another possible catch thread design that can be used is equivalent tospool type interlocking locking thread described in FIG. 2 of the abovequoted BS. As adapted to join two abutting fabric edges, this wouldsimply be a thread which passed alternately through edge weft loops of afirst fabric edge and then a second fabric edge.

Where two abutting fabric edges are joined by the catch thread, theseedges may be provided (i) by longitudinal edges of a single fabric piecewrapped into a tubular configuration, (ii) by edges of two or moreseparate fabric pieces, arranged adjacent to each other, or (iii) byedges of two or more previously separate fabric pieces which have beenfastened to each other by some means, preferably by some mechanicalpenetrating means such as sewing, stapling or riveting.

For a woven fabric joined by a catch thread, preferred materials for thefabric are as follows: weft thread--heat shrinkable polyolefin, e.g.high density polyethylene, having a shrink ratio of 10×; warpthread-polyethylene terephthalate; catch thread--polyolefin, e.g. highdensity polyethylene, but of smaller diameter than the high densitypolyethylene of the weft fibers of the fabric.

Other suitable materials that can be used include the following: nylon;acrylics; aromatic polyamides; polypropylene; cotton; wool; rayon(regenerated cellulose); cellulose acetate; pyrolised polyacrylonitrile;carbon; homopolymer, modified polymer, or copolymers of acrylics;fluoropolymers; and glass.

Another fabric structure that could be used is a weft insertion warpknitted fabric with heat shrinkable fibers as the weft inlay. In oneembodiment according to the invention, the heat shrinkable fibers areabout 0.4 mm diameter HDPE, the majority of the knitted loops in thewarp direction are two fold, 167 Tex, Kevlar 29 (Trademark of Dupont)with a single wale (volume of stitches) of three fold 16.7 Tex,polyethylene terephthalate as the line of weakness. In a particularlypreferred embodiment, the structure is sewn or stapled around anexpandable object such as an airbag. When sewn, a strong lock stitch ofKevlar (five fold, 22 Tex) is preferably used so that the fault line isin the 3 fold/16.7 Tex Polyethylene terephthalate.

Yet another fabric structure that could be used is a flat woven sheet.In a flat woven sheet a line of weakness may be provided by a sewingthread stitched to make the flat sheet into a tube. A single threadchain stitch of two fold 22 Tex Nomex (trade name) with a breakingstrength of 10 Newtons may be used to sew the flat woven tube into asheet.

As used herein "Tex", which is the I.S.O designation for Linear Densityof Textile strands, yarns, fibers etc., is the weight in grams of 1000metres of product.

Where the shrinkable cover is a composite cover comprising fibrouscomponents and a polymeric matrix, then a combination of the abovedesign features referred to for plain polymeric, shrinkable covers andfor fabric shrinkable covers, may be used to provide the desired regionof weakness.

Fabric shrinkable covers, or composite covers containing fibers orfabrics are particularly advantageous for use as shrinkable air bagcovers, since the covers are particularly resistant to tearing onshrinkage.

Tear resistance of the shrinkable cover is particularly important forseveral reasons. First the cover is desirably installed around the airbag, or other expandable object, to compact it, by shrinkage, preferablyby heat. It is therefore important that the cover does not tearprematurely during this shrinkage process. Secondly, for someapplications, e.g. some airbag applications, it is important to anchorthe air bag to the vehicle frame, so that the air bag does not fly intothe vehicle when deployed, since this could injure the occupants. Byproviding a tear resistant cover, this cover may conveniently beanchored to the vehicle frame by a mechanical penetrating fixing means,such as a bolt, rivet, staple or screw or the like, passing through theair bag cover. For these reasons, therefore, fabric shrinkable covers,or fibrous-based shrinkable covers are particularly preferred, since anysplits in the fabric around the mechanical penetrating fixing means arenot propagated during installation by shrinkage (e.g. heat shrinkage fora heat shrinkable cover) of the cover. The good tear resistancecapabilities of fabric materials are known.

Anchorage of an airbag and also its cover, at various points isimportant to avoid injury to passengers. As an example, a side impactairbag designed to protect a front passenger, and extending diagonallyacross the window of the front door, may be anchored at two points, ateach end of an airbag encased in a generally tubular cover. As anotherexample an airbag deploying vertically downwards in a sheet likeconfiguration along the entire side length of a vehicle may be anchoredto the top of the side frame of the vehicle, together with itsencapsulating cover, at several (e.g. four to ten) points along itslength.

In preferred tubular fabric-based covers according to the invention, adraw thread may be included, within the tubular cover, during fabricmanufacture. This draw thread may conveniently be used to pull elongateexpandable objects easily into the tubular cover.

As mentioned above, the shrinkable cover is preferably shrunk around theair bag to compact the air bag. During this operation, the whole, oronly part of the shrinkable cover may be shrunk. For a heat shrinkablecover, for example, it may be desirable, to heat only those regions awayfrom the region of weakness of the heat-shrinkable cover.

Embodiments of the present invention will now be described, by way ofexample, wherein:

FIG. 1 is a perspective view of a side impact vehicular air bag;

FIG. 2a is a perspective view of the air bag of FIG. 1 encapsulated in acover according to the invention, and bolted to a vehicle frame.

FIG. 2b is a perspective view of an airbag cover that can be used inplace of that shown in FIG. 2a;

FIG. 3 is an enlarged view of Section III taken through the region ofweakness of the article shown in FIG. 2a.

FIG. 4 is a perspective view showing inflation of the air bag of FIG.2a, and consequent rupture of the cover encapsulating the airbag of FIG.2;

FIG. 5 is a cross-sectional view through a front impact air bagencapsulated by an article according to the present invention; and

FIG. 6 is a plan view through the article used in FIG. 5.

Referring now to the drawings, FIG. 1 shows a typical design of a sideimpact air bag 1 designed to protect a front seat passenger. It isgenerally elongate in structure (as indicated by the dotted lines), andtypically measures about 1.2 metres in length by about 20 cm in width.The air bag is bonded along lines 3 to form separate channels 5. Eachchannel 5 is in communication with a gas chamber 7 (not shown). Gas isreleased into channels 5 in the event of an accident to inflate the airbag. The air bag 1 is also provided with a flange 7 along one side, forconvenient attachment to the vehicle frame. Other side impact airbags,designed to extend along the entire length of the passenger compartment,are typically larger, e.g. about 2.4 metres in length by about 50 cm inwidth. Their design is otherwise similar to that described above.

FIG. 2a shows the air bag 1 of FIG. 1 folded into a concertina shape,and encapsulated by a shrinkable tubular fabric article 9 according tothe present invention. The fabric article 9 is woven and tubular. Itcomprises multifilament polyethylene terephthalate fibers in the warp,running longitudinally along the tube, and heat shrinkable high densitypolyethylene monofilament fibers in the weft, extending generallycircumferentially. The shrinkable fibers have a recovery ratio of about8:1 to 10:1 (8×to 10×. Extending along the tubular article 9, in thewarp direction, is thin catch thread 13, comprising a shrinkable highdensity polyethylene yarn. The interengagement of the catch thread 13,and the weft fibers of fabric 9, is shown and described in more detailwith reference to FIG. 3.

The warp fibers of the fabric of cover 9 immediately adjacent the catchthread 13 have been identified by the reference numeral 11 foridentification purposes. The catch thread 13, and/or the warp fibers 11,and/or further warp fibers adjacent warp fibers 11 may be a differentcolor from the other fibers of fabric, in order to make the region ofweakness provided by catch thread 13 readily visible.

The fabric article 9 has been heated to shrink it around the air bag 1,to compact the air bag into its concertina configuration, and tomaintain it in that configuration. Typically, once shrunk, there isbetween 50-80% unresolved recovery in the cover, i.e. if allowed torecover freely the cover would recover between 50 and 80% more. Theencapsulated air bag 1 is secured to part of the vehicle frame 15 bybolts 17 which pass through the fabric cover 9 surrounding the airbag 1. The fabric area through which bolts 17 pass may be reinforced,e.g. by increasing the fiber density (not shown).

FIG. 2b shows an alternative flat construction of fabric pieces 9", 9"'joined by a warp catch thread 13. The free edges 14 of the sheets 9",9"' comprise an increased number of fibers in the warp, longitudinaldirection 1. This reinforces the edge regions 14. As in FIG. 2a, thewarp fibers immediately adjacent the catch thread 13 are indicated byreference numeral 11. The sheets 9", 9"' may be wrapped into a tubularconfiguration and bolts 17 passed into vehicle frame 15 through the edgeregions 14 of the sheets 9", 9"' in a manner similar to that describedin FIG. 2a. Edges 14 may be brought together in a lap configuration, oras a flange extending outwardly or inwardly of the tube so formed.

The properties of the fabric making up the preferred design of the airbag cover 9 of FIGS. 1 and 2a are as follows:

    ______________________________________                                                                                Break load                            Weave  Yarn                       Elong.                                                                              (room                                 Design Sett     Material   Thickness                                                                            to break                                                                            temp)                                 ______________________________________                                        Plain  Warp     Polyethylene                                                                             16.7 tex                                                                             35%   273 N/cm                              weave  8 ends/cm                                                                              Terephthalate                                                                            7 ply        in warp                               (tubular)                                                                            on each  (multifilamnt)          direction                                    surface                                                                       Weft     HDPE       47 tex 20%   178 N/cm                                     5 double (monofilamnt)                                                                            0.25 mm      in weft                                      insertions          diameter     direction                                    per cm on                                                                     each                                                                          surface                                                                ______________________________________                                    

FIG. 3 shows in enlarged view the interengagement of the catch thread 13with the weft fibers 19 of the fabric cover 9. The interengagement showncorresponds to the needle loom selvedge system of the type described inFIG. 2 of BS 7141, adapted for joining two fabric edges. In FIG. 3,general warp fibers are indicated by reference numerals 21, and the warpfibers immediately adjacent the catch thread 13 are indicated byreference numeral 11 as in FIGS. 2a and 2b.

FIG. 4 is a schematic cross-section view showing the deployment of theair bag 1 from the cover 9 of FIG. 2a, in the event of an accident. Gasis introduced into chambers 5 of the air bag, inflating them, so thatthe air bag volume exceeds that of the containing fabric cover 9. Thecatch thread 13 is therefore broken by the expanding air bag, and thelongitudinal edges 11 of the fabric sleeve are burst apart. The cover 9therefore splits along edges 11 to allow the air bag 1 to open into thevehicle. The air bag 1 remains anchored to the vehicle 15 by the bolts17, preventing the air bag flying around the vehicle and injuringpassengers.

FIG. 5 shows an alternative arrangement where a front impact air bag 1'is retained by a covering sheet 9' secured to part of the vehicle 23,and to the back housing 24 of gas chamber 26, by bolts 25. As shown inFIG. 6, the sheet 9' is provided with a line of weakness 27 ashereinbefore described. The sheet is also provided with apertures 29 forreceiving the bolts 25. The fabric sheet may be similar to that shown inFIG. 2b, with the bolts 25 passing through the reinforced edges 14.

The sheet 9' is installed on the air bag 1' to compact the air bag, byheating a strip 31 only of the sheet 9' Recovery during installation ispreferably limited to a maximum of 5% to reduce the risk of tearing thesheet 9' during installation.

In the event of an accident, gas from chamber 26 inflates air bag 1',the sheet 9' tears along fault line 27, and the air bag is thusdeployed.

What is claimed is:
 1. A fabric cover, which is shrinkable at least inpart, for encapsulating at least part of an expandable object, the coverbeing provided with a predetermined region of weakness, which willrupture when the said object expands by greater than a predeterminedamount; wherein the cover comprises one or more woven fabric pieces, andthe predetermined region of weakness is provided by a catch threadinterengaging weft loops at abutting edges of a single wrapped fabric;or two adjacent separate abutting woven fabrics.
 2. A cover according toclaim 1, comprising a composite sheet containing a fabric and apolymeric matrix.
 3. A cover according to claim 1, wherein the fabric isheat shrinkable.
 4. A cover according to claim 3, wherein the fabriccomprises polymeric fibers that are heat shrinkable.
 5. A coveraccording to claim 1 which has a shrinkage ratio of at least 1.5:1.
 6. Acover according to claim 1 which has a thickness of at least 0.08 mm. 7.A cover according to claim 1, which has a recovery force of at least 0.1MPa.
 8. A cover according to claim 1, wherein the catch thread has atensile strength, at 23° C., lower than the tensile strength at 23° C.of the weft fibers of the or each fabric.
 9. A cover according to claim1, wherein the catch thread has a diameter less than the diameter of theweft fiber or each fabric.
 10. A cover according to claim 1, wherein theregion of weakness is a different color from the surrounding region ofthe cover.
 11. A cover according to claim 1, wherein a section of thecover immediately adjacent the region of weakness is a different colorfrom the remainder of the cover.
 12. A cover according to claim 1, incombination with an expandable object, which can be furled or folded orotherwise compacted in size.
 13. A cover according to claim 12, whichhas been shrunk around the object to compact the said object.
 14. Amethod of encapsulating at least part of an expandable object, themethod comprising installing a shrinkable cover according to claim 1over the said part of the expandable object to restrain the said objectin a first, compressed, state, the cover being provided with apredetermined region of weakness, which will rupture when the saidobject expands, from a first volume, by greater than a predeterminedamount.
 15. A fabric cover, which is shrinkable at least in part, forencapsulating at least part of an expandable object, the cover beingprovided with a predetermined region of weakness, which will rupturewhen the said object expands by greater than a predetermined amount;wherein the cover is tubular and comprises a woven fabric, thepredetermined region of weakness being provided by a catch thread whichextends as one of the warp yarns extending in the longitudinal directionof the tubular cover, which catch thread interengages weft loops whichextend in the circumferential direction of the tubular cover, and whichloop back on themselves around the said catch thread.